The present invention relates to pattern defect inspection methods and apparatus using a laser beam as illumination light, mainly for inspecting and observing defects of micro patterns or foreign matter contamination occurring in manufacturing processes for semiconductor devices and flat panel displays.
Circuit patterns tend to become finer and smaller as semiconductor devices become more highly integrated. Smaller and finer circuit patterns have spurred a demand for higher resolution when inspecting for defects of circuit patterns that have been formed on semiconductor wafers by photolithographic processes using photomasks or reticles. One technique for enhancing resolution when detecting pattern defects involves the use of illumination light on shorter wavelengths from visible light to ultraviolet light. Mercury lamps and xenon lamps, for example, have been conventionally used as illumination light sources, while only the required wavelengths from the various line spectra emitted from these lamps are optically selected and utilized.
In addition to high resolution, pattern defect inspection systems also require high intensity illumination to shorten the inspection time. Illumination from a typical light source lamp contains only a few line spectra in the ultraviolet region. A larger size lamp with higher power must be used to obtain a high intensity sufficient for high-speed pattern inspection, but this results in the problem of lower lighting efficiency. Even if high intensity illumination is obtained by utilizing a wider spectral band, there is the problem that correcting the chromatic aberration of optical systems used for pattern inspection is difficult.
Optical aligners of the type used in semiconductor device manufacturing also require the same high resolution as a pattern defect inspection apparatus, so optical aligners equipped with a KrF excimer laser that emits light at a 248 nm wavelength are mainly used. Optical aligners using an ArF laser that emits an even shorter 193 nm wavelength have also been developed. However, these excimer lasers are large in size and use fluorine gases that are harmful to the human body, so specific safety measures must be implemented.
Recently, a great deal of attention is being focused on solid-state YAG lasers as another type of ultraviolet laser. YAG lasers can generate a third harmonic (355 nm wavelength) or fourth harmonic (266 nm wavelength) by wavelength conversion when the laser beam is passed through a nonlinear optical crystal. This has led to the development of compact, easy to handle ultraviolet lasers. These compact and easy to use ultraviolet lasers are highly effective for use in a pattern inspection apparatus.
Laser beams have superior coherence, but this causes enhancement and attenuation in the light flux when they are used to illuminate a sample, and such illumination produces an interference fringe on the sample. In a pattern inspection apparatus using a laser, as disclosed in Japanese Patent JP-A No. 271213/1999, a light beam emitted from a laser light source is guided into a fly-eye lens (micro-lens array) to form a multi-spot light source. This multi-spot light source is focused so as to strike a sample under test, so that the sample is uniformly illuminated with light. The intensity of the light reflecting from the sample is then detected with a charge integration type of CCD line sensor.
The aforesaid pattern defect inspection apparatus using a laser has the following problems.
The light beam emitted from the laser is transformed into a multi-spot light source by a fly-eye lens and is focused by a condenser lens so as to illuminate the entire area of the sample under test. The incident angle of the illumination light on the surface of the sample under test is determined by the focal positions of the fly-eye lens and the condenser lens.
Multi-layered circuit patterns are fabricated on the surface of the sample (semiconductor wafer) by a semiconductor wafer process. During this process, upper layer patterns are formed on lower layer patterns with a thin film being formed between the patterns. Thus, pattern inspection is performed mainly on the upper layer patterns; however, when the surface of the sample is illuminated with light, the light reflected from the sample contains light components reflecting from the surface of the thin film and also light components reflecting from points inside of the thin film. Thus, the intensity of light reflecting from points inside of the thin film changes according to the thickness of the thin film.
Now we will discuss how the intensity of reflected light changes in cases where a thin film, such as an insulating film, is formed on the surface of a sample. A typical interference model is shown in FIG. 11. Here, the wavelength of illumination light 37 is set as λ, the incident angle of the illumination light 37 relative to a line perpendicular to the surface of the sample is θ, the refractive index of the air layer 34 is n0, the thickness and refractive index of the thin film 35 are t1 and n1, respectively, and the refractive index of the substrate 36 is n2. If the intensity of light 38 reflected from the surface of the thin film 35 is set as r01, and the intensity of light 39 reflected from the substrate 36 after passing through the thin film 35 is r12, then the composite reflected light can be defined as R. These factors can be theoretically modeled as Fresnel equations and expressed by the following equations 1 to 4.
                    X        =                                            4              ⁢              π              ⁢                                                          ⁢              n1t1                        λ                    ⁢          cos          ⁢                                          ⁢          θ                                    (                  Eq          .                                          ⁢          1                )                                          r          01                =                              n1            -            n0                                n1            +            n0                                              (                  Eq          .                                          ⁢          2                )                                          r          12                =                              n2            -            n1                                n2            +            n1                                              (                  Eq          .                                          ⁢          3                )                                R        =                                            r01              2                        +                          r12              2                        +                          2              ⁢              r01r12cos              ⁢                                                          ⁢                              (                χ                )                                                          1            +                                          r01                2                            ⁢                              r12                2                                      +                          2              ⁢              r01r12cos              ⁢                                                          ⁢                              (                χ                )                                                                        (                  Eq          .                                          ⁢          4                )            An example of calculated results is shown in FIG. 12, wherein the horizontal axis represents the thickness of the thin film 35 and the vertical axis represents the composite light intensity R. Changes in the composite light intensity versus the film thickness, when plotted, result in waveform 40.
However, when a laser beam is used to illuminate a sample, in order to ensure an adequate illumination sigma (s) (explained later in “Description of The Preferred Embodiments”), the laser beam must be scanned, for example, when input onto an objective lens, since lasers are point light sources. Inputting the laser beam onto the objective lens, while it is being scanned, shifts the incident angle relative to the surface of the substrate and changes the irradiance of the laser beam striking the substrate. Whether the incident light angle is large or small causes a difference in the reflected light intensity, which also varies according to the thickness of the thin film 35, as shown in FIG. 8. If the sample is inspected under such conditions, changes in reflected light intensity, due to the thickness distribution of the thin film 35, appear as changes in the brightness, causing lower detection sensitivity.